1. Field of the Invention
The present invention relates generally to virtual sensors and, more particularly, to a means and method utilizing a neural network for estimating helicopter airspeed at speeds below about 50 knots using only fixed system parameters (i.e., parameters measured or determined in a reference frame fixed relative to the helicopter fuselage) as inputs to the neural network.
2. Brief Description of Related Art
Helicopters are designed for a wide variety of missions including anti-submarine warfare, vertical replenishment, and search and rescue missions. Although helicopters routinely operate at forward airspeeds above 100 knots, such missions require that a large portion of flight time be conducted in the low airspeed flight regime (i.e., airspeeds below about 50 knots). Because flight in the low airspeed regime requires increased power, accurate low airspeed data is needed to maintain control margins. Low airspeed data is needed by pilots flying instrument approaches in order to maintain critical control authority, particularly in connection with tail-rotor effectiveness. On attack helicopters, low airspeed information is critical to accurate weapons firing solutions. In addition, high vibratory loads can occur in some low airspeed maneuvers resulting in fatigue damage accumulation in flight critical components. Technology for monitoring the safe life remaining on such flight critical components has been developed through helicopter usage monitoring and flight regime recognition techniques, e.g., Health and Usage Monitoring Systems (HUMS). Normally, information from multiple sensors must be examined collectively to make diagnostic and prognostic decisions. However, the success of HUMS technology in the low airspeed regime is dependent on accurate low airspeed information. Without correct low airspeed information, usage monitoring algorithms cannot recognize the low airspeed maneuvers and, therefore, may not register critical fatigue accumulation data.
Due to inaccuracy associated with use of traditional pitot-static probes in a low airspeed environment, as well as with interference generated by the main rotor downwash, instrumentation for accurately measuring airspeed and sideslip angle in the low airspeed regime is generally lacking. Thus, although accurate low airspeed information is needed by pilots and monitoring algorithms, it is not available using traditional methods of measuring airspeed and sideslip angle.
Development of a measurement system that accurately estimates low airspeed and sideslip angle has long been a difficult challenge. Interest in low airspeed measurement began in the 1950s when preliminary concepts were developed and flight tested. These concepts involved mounting probes above the rotor hub as well as in the wake beneath the rotor. Since the 1950s, these concepts have been refined and a variety of low airspeed sensor designs have been flight tested. One such system employs two venturi tubes on opposite ends of a rotating arm installed above the rotor hub to measure true airspeed magnitude and direction, e.g., LORAS (Low Range Airspeed System) produced by the Pacer Company of the United States. The differential pressure between the two sensors is used to calculate the airspeed and sideslip angle. Such systems, however, require slip ring assemblies or some other means of transferring data from the rotating reference frame of the rotor to the fixed (i.e., nonrotating) reference frame of the fuselage. Another approach involves a sensor designed to be mounted under the rotor wherein the nature of the wake is used to determine helicopter airspeed, e.g., LASSIE (Low Air Speed Sensing and Indicating System) produced by the GEC Company of England. This system uses a pitot-static probe which can rotate about 360.degree. to provide airspeed and sideslip angle information. However, the flow environment under the rotor system is complex and empirical methods are used to linearize the output. Several other techniques, including those using ultrasonic transmission times and shed vortex characteristics, have been proposed for deducing low airspeed and sideslip angle information.
The search for an effective low airspeed sensor has long been a difficult challenge for the helicopter R&D sector. Few proposed solutions have made it into use. Most proposed low airspeed measurement systems are externally mounted and require transferring information from a reference frame rotating with the rotor to a reference frame fixed relative to the helicopter fuselage (i.e., a helicopter fixed system of coordinates XYZ originating in the helicopter fuselage). Due to the mechanical complexity, expense, and increased drag introduced by proposed low airspeed measurement systems, most helicopters are not equipped with low airspeed sensors. Moreover, in many cases, physical sensors cannot be affordably and reliably applied in an operational environment on military helicopters. Thus, the vast majority of commercial and military helicopters in use today do not have an airspeed system that can accurately measure airspeed below about 50 knots even though this is within the flight regime of the helicopter. Generally, investment in low airspeed measurement equipment is reserved for those aircraft with a critical low airspeed mission. Consequently, there is a need for a simple, low cost means and method for determining low airspeed and sideslip angle experienced by the helicopter.